German Patent Application No. DE 101 39 142 describes an exhaust gas aftertreatment unit for an internal combustion engine in which the concentration of a urea-water solution in a tank is determined to permit accurate metering of the urea-water solution into the exhaust gas area of the internal combustion engine. The urea-water solution is a reagent that acts as a reducing agent in an SCR catalytic converter, where it reduces the nitrogen oxides contained in the exhaust of the internal combustion engine. In a first reaction step, the urea present in the urea-water solution reacts with water (undergoing hydrolysis) to form ammonia and carbon dioxide. In a second reaction step, NO and NO2 react with ammonia to form nitrogen and water. The flow rate of the urea-water solution is adjusted by a metering valve and must not exceed an upper limit or drop below a lower limit. If it drops below the lower limit, the SCR catalytic converter is ineffective, and if it exceeds the upper limit, a breakthrough of ammonia occurs.
An object of the present invention is to provide a method for operating a metering valve and a device for performing the method, making it possible to maintain a specified flow rate of a reagent to be introduced into an exhaust gas area of an internal combustion engine.